Hybrid vehicles are well known, and typically comprise an internal combustion engine and an electric motor powered by a battery. The vehicle may be powered by the engine, by the motor, or by a combination of engine and motor. In the latter case sophisticated controls have been developed to ensure that the wheel torque demanded by the driver, and any additional torque to charge the battery, can be delivered by proportioning the torque delivered by the engine and motor so as to meet a pre-determined target, which may be based for example on one of minimized fuel consumption, maximized range, and maximized vehicle performance. The optimum state of charge of the battery typically varies according to the pre-determined target.
The vehicle battery may be recharged by the usual generator of the engine or by the motor acting as a generator; in the latter case the motor may be driven by the wheels (regenerative braking) or by the engine.
In order to provide familiar feedback to the driver, who may be more familiar with the characteristics of a more standard non-hybrid vehicle, it is desirable to provide a substantially consistent degree of deceleration on overrun—overrun is defined in this specification as slowing of the vehicle without assistance of braking, typically at a substantially zero position of the accelerator pedal.
In a hybrid vehicle, providing consistent deceleration is somewhat problematic. If the engine is running and fully engaged with the driveline, a hybrid vehicle can exhibit a normal deceleration in response to foot-off of the accelerator pedal provided that additional negative torque from the motor is avoided.
However if the engine is disconnected from the driveline and the vehicle is operating in electric-only mode, retardation can be provided solely by the motor (acting as generator). In this circumstance it may be possible to determine empirically the deceleration torque applied by the engine (the torque to overcome friction and pumping losses) and cause the motor to provide the equivalent deceleration torque by acting as a generator (and thus recharging the battery).
A difficulty arises if the battery is fully charged, because the motor control system may prevent the motor acting as a generator, so that the normal degree of deceleration cannot be achieved.
In addition, a negative torque (deceleration) target based on friction and pumping losses of the engine takes no account of external vehicle conditions which are apparent to the vehicle driver, such as gradient, speed ratio of the transmission and vehicle mass; changes in these conditions will cause the vehicle driver to expect a different deceleration profile.
Provision of consistent deceleration becomes more difficult when the motor (as generator) provides only a portion of the negative torque, and the engine provides the remainder via a slipping clutch.
Accordingly a better method and means of achieving consistent overrun is required.